Roy Lewallen wrote:
I can easily be mistaken, but I don't believe the NBS Yagi designs were
ever promoted as being optimal in any way. Their purpose, I believe,
was to provide a set of easy-to-duplicate designs whose gains were well
documented (although I understand an error was made in measuring the
originals) and which anyone could construct as reference antennas with
known gain.


They were in fact described as "optimum designs" in the original paper,
but it's clear that claim was made subject to a number of reservations.
Most of the hype came from antenna manufacturers who subsequently picked
up the designs from the published reports, and misused the "NBS" name to
boost the credibility of their own products.

With hindsight, the NBS designs were really quite good, and their
forward gain is still competitive with more modern designs of the same
boom length. But modern yagi designs are generally better, because there
have been a further 35-40 years of development and optimization. In
particular, the last 10 years have benefited from the availability of
computer modeling techniques - you can now do more good work in a few
evenings than the NBS program achieved in as many years.

As a result of this development, you can now have a better combination
of features - for example, cleaner patterns, wider bandwidth and/or
greater tolerance to dimensional errors, and easier impedance matching -
and keep the good forward gain as well.

Many modern designs have been developed as complete 'families', with
simple design rules that let you add or remove elements (changing the
boom length accordingly) to create new yagis, each of which will be
close to optimum for its boom length. The NBS yagis don't have that
'family' feature - each one is an individual design, and attempts to
adapt them have generally not been successful. This lack of adaptability
is a direct consequence of the original decision to use the same spacing
between all elements; that's why I described it as an "evolutionary
dead-end".

It costs exactly the same to build a good, modern yagi as it does to
build an older, inferior design - the only differences are in *where*
you apply the hacksaw and the drill. Therefore there's not much point in
building an inferior design... which is what the NBS yagis have now
become.

For many years, the greatest value of the NBS yagis was that they had
accurate gain measurements, so they could be used as reliable benchmarks
in antenna gain shootouts. (The known error in the gain measurements
applies to the 2-element yagi only.) Even that use has now been
overtaken by computer modeling.

In summary, the NBS yagis deserve respect for their major contribution
to the art and science of yagi design, but they are now mainly of
historical interest.

For examples of modern yagis, and tips on construction, see the 'VHF/UHF
Long Yagi Workshop' on my website.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek